1. Field of the Invention
The present invention relates to an apparatus and a method for obtaining spectral characteristics.
2. Description of the Related Art
One of the important technical tasks of an image forming apparatus (e.g., printers, and printing apparatuses and like) is management of color tone including, for example, management of color stability and color reproduction. Recent image forming apparatuses have a spectral device (e.g., spectrometer) mounted thereon for performing color management. Within the recent image forming apparatuses, measurement of colorimetric values (e.g., XYZ or L*a*b* defined by CIE (International Commission On Illumination)), inspection of color tone of printed material, and adjustment of an image forming process are performed based on spectral reflectance from light diffused from a surface of a printed material measured by the spectral device.
The spectral device, which measures visible light, can output, for example, a discretized value of 31 or more bands detected from a light having a wavelength band ranging from 400 nm to 700 nm at intervals of 10 nm. In order to obtain optical intensity signals by temporally and spatially dividing diffuse reflection light to 31 or more, a substantial amount of time is required for measuring the spectral reflectance. Therefore, it is difficult to use the spectral device for an image forming apparatus performing high speed printing because detection speed is insufficient for performing inline measurement on an image at a rate corresponding to the printing speed.
Thus, there is a method of using a spectral device for detecting optical intensity signals from a comparatively small wavelength band of 3-16 (referred to as multi-band) in a case of measuring spectral reflectance from a measurement target (target to be measured) exhibiting a comparatively moderately changing spectral reflectance distribution. Thereby, spectral reflectance can be estimated from the measurement target according to the result of the detection of optical intensity signals (see, for example, Japanese Laid-Open Patent Publication No. 2009-219006). With this method, spectral reflectance can be estimated with high precision in a case where the measurement target is limited to an object from which statistical spectral reflectance data (statistical data pertaining to spectral reflectance) can be obtained beforehand (e.g., print image that can have its colors reproduced by combining 4 types of color material). Further, with this method, the time for detecting optical intensity signals can be reduced because the number of optical intensity signals to be detected is small. Accordingly, this method can be applied to a field requiring high speed measurement such as inline measurement of a print image.
It is preferable for the light source of the above-described spectral device to have high stability/long life-span in addition to having high luminance and high efficiency such as a white LED. However, the white LED is formed of an LED that emits blue light and a fluorescent material that absorbs the blue light and emits yellow light. In the spectral distribution of the white LED, the wavelength bands other than the two peaks of blue and yellow exhibit a low spectral luminance, and the white LED does not exhibit a uniform luminance throughout the entire visible wavelength range. Therefore, in a case where a light source such as white LED is used in the above-described spectral device for measuring a color with high precision, the precision of measuring a particular color may be significantly degraded. Further, in a case of performing high speed printing, it is difficult to change light exposure time with respect to bandwidths in accordance with the light quantity of each of the bandwidths. Therefore, the precision of estimating the spectral reflectance of a particular bandwidth may be significantly degraded.
Accordingly, Japanese Laid-Open Patent Publication No. 2009-219006 proposes a spectral reflectance deriving apparatus for deriving a spectral reflectance of a reproduction output object that is close (similar) to the spectral reflectance of a reproduction target even with different light sources by using a spectral reflectance decomposition method and a spectral reflectance estimation model and assuming that the color difference between the reproduction target and the reproduction output object is almost 0 where a particular light source is used in a printing apparatus or the like.
However, with the spectral reflectance deriving apparatus of Japanese Laid-Open Patent Publication No. 2009-219006, it becomes necessary to perform decomposition on the spectral reflectance target by using the spectral decomposition method, so that error can be minimized. This makes it difficult to estimate the spectral reflectance of the measurement object with high precision and high speed.
Although the above-described difficulty may be resolved by a method of using a light source capable of uniformly illuminating the entire wavelength range to be measured or a method of using a sensor capable of detecting a particular bandwidth with high sensitivity, these methods increase, for example, manufacturing cost.